Isorunning-counterrunning developer station for an electrophotographic means

ABSTRACT

An isorunning-counterrunning developer station comprises a plurality of isorunning developer drums moving isodirectionally with the charge image carrier and one counterruning developer drum. A developer mix is offered to the first isorunning developer drum via a paddle wheel proceeding from a supply chamber and is conducted from the first isorunning developer drum to the counterrunning developer drum via further isorunning developer drums. An exposure means composed of a transparent protective tube having a LED strip contained therein is provided above the developer drum, this exposure means being in turn followed by a carrier stripper drum.

BACKGROUND OF THE INVENTION

The invention is directed to a developer station in anelectrophotographic means for the development of charge images appliedto charge image carriers with the assistance of a developer mix, wherebythe developer mix is taken from a supply chamber in a developer stationby a transport drum and is transported to developer drums that rotate inopposite directions relative to one another and ink the charge imagecarrier.

In copier equipment technology and in non-mechanical fast data printersthat operate based on the principle of electrophotography, charge imagesare generated on a charge image carrier, for example on aphotoconductive drum, and are subsequently inked with a colored powder,toner in a developer station. The toner images are subsequentlytransferred onto normal paper given employment of a photoconductive drumand are fixed there.

As a rule, a two-component developer is employed for developing, thisbeing composed of ferromagnetic carrier particles and of colored tonerparticles. The developer mix, for example, is conducted past the chargeimage on the charge image carrier with a magnetic brush arrangement, thetoner particles adhering to the charge image as a result ofelectrostatic forces. The magnetic brush arrangement is thereby composedof a rotatable hollow cylinder in whose interior a plurality of rows ofstationary permanent magnets are arranged. A plurality of magnetic brusharrangements can be provided in one developer station. For example, onemagnetic brush arrangement can serve the purpose of transporting thedeveloper mix past the charge image carrier, referred to below asdeveloper drum. A further magnetic brush arrangement can be employed inorder to transport the developer mix out of the inside of the developerstation to the developer drum. Such a magnetic brush arrangement or anyother arrangement that effects such a transport of developer mix isthereby referred to as transport drum below. Developer stations whereindeveloper mix for inking the charge images on the charge image carrierwith the assistance of the magnetic brush principle are employed aredisclosed by German Patent 31 19 010 and corresponding U.S. Pat. No.4,461,232.

When developer drums are operated such that the developer mix is appliedin the moving direction of the charge image carrier, one speaks ofisorunning developer drums or co-running developer drums. When thedeveloper mix is applied opposite the moving direction of the chargeimage carrier with the assistance of the developer drums, then suchdeveloper drums are referred to as counterrunning developer drums.

Developer stations that contain both counterrunning as well asisorunning developer drums are disclosed, for example, by US-A-3,912,388and by US-A-3,881,446. The developer drums additionally have a meteringmeans allocated to them that is composed of an adjustable blade and thatserves the purpose of impressing a defined height or, respectively,thickness on the carpet of mix on the developer drums. The developer mixis thereby simultaneously offered to both drums from the supply space.

Developer stations having isorunning developer drums as disclosed, forexample, by GB-A-1 524 543 have good printing results given line-likepatterns such as, for example, characters. The inking is unsatisfactorygiven full surfaces such as occur, for example, in graphic illustrationsor given black bars as needed, for example, when printing bar codes.

Numerous attempts have been made in order to be able to satisfactorilyaccomplish the full-surface inking.

Thus, a better full-surface inking without modification of the developerstation can be achieved when the toner and/or the carrier material inthe developer mix is changed. In all such instances, however, aconsiderable reduction of the useful life of the mix has hitherto alsoderived, this being a great disadvantage because of the higher costs formaterial consumption and maintenance connected therewith.

A further possibility of improving the full-surface inking is comprisedin switching from the reversal development method wherein the dischargedregions of the charge image carrier are inked to the direct developmentmethod standard in copier technology wherein the charged regions areinked. Given direct development, however, the majority part of thesurface of the charge image carrier must usually be exposed. Thislargely excludes the employment of specific character generatorprinciples such as, for example, LED lines or the employment of laserdiodes because of the thermic problems connected therewith. However, itis precisely these two character generator principles that come intoconsideration when high point grid density given high printing processspeed are required.

Further attempts for improving the full-surface inking have shown thatno uniform surface inking can be achieved with a single developer drumgiven the established marginal conditions (composition of the toner mix,speed). On the contrary, wash-out effects at the trailing edge of thefull surfaces given isorunning development or, respectively, at theleading edge of the full surface given counterrunning development areobserved therein, these having an especially disturbing effect incritical image patterns, for example inverse printing.

Disturbances in the printer format, particularly light spots, form afurther problem in such developer stations, these light spots derivingtherefrom that the carrier particles contained in the developer mix areentrained by the photoconductive layer of the photoconductive drum up tothe transfer station and deteriorate the transfer printing event. Thestandard magnetic carrier stripper drum is not in a position by itselfto reliably avoid this disturbance.

SUMMARY OF THE INVENTION

It is an object of the-in to fashion a developer station of the speciesinitially cited such that a high printing quality given high processspeed is possible upon employment of a developer mix having a highuseful life of the mix.

In a developer station of the species initially cited, this object isachieved in that the developer station is fashioned as anisorunning-counterrunning developer station wherein the developer mix isinitially offered to the photoconductive drum via a plurality ofdeveloper drums moving isodirectionally, whereby the last developer drumis then fashioned as a developer drum that is movedcounter-directionally. The circumferential speed of the isorunningdeveloper drums is thereby noticeably higher than the circumferentialspeed of the charge image carrier. Even given employment of reversaldevelopment methods, this measure achieves an intense and uniform inkingof the full surfaces given a high printing speed of, for example, 0.4m/sec and more.

In an advantageous embodiment of the invention, an illumination meansfor the charge image carrier is provided following the developer drumsin moving direction of the charge image carrier. This illumination meanscan be surrounded by a transparent, rotating protective drum thatprevents the contamination of the LED strip or light-emitting foilserving as exposure elements.

After the actual development, this illumination means levels the chargeimage in that the highly charged and non-toner-covered regions of thecharge image carrier are illuminated and, thus, discharged. Therewith,first, the adhesion of the "negative" charge particles to thephotoconductor regions that were not exposed in character-dependentfashion is reduced, so that they can be stripped from the charge imagecarrier by a following carrier stripper drum and can be returned intothe developer station. The memory effects on the charge image carrierdue to charge images that are not completely quenched and that disturbthe printed format can also be prevented by the illumination means.

The spacing of the illumination means from the counterrunning developerdrum is adapted such that the surface of the protective tube composed ofplexiglass is continuously cleaned of developer mix and the light exitis only slightly attenuated by adhering toner dust.

Given a further, advantageous embodiment of the invention, a suctionmeans is provided between the plexiglass protective tube of theillumination means and the carrier stripper drum, this suction meanssuctioning the free toner mix--that is not bonded to the charge imagecarrier--off, so that it cannot be entrained upward out of the developerstation.

The transfer of the carpet of developer mix from the last isorunningroller onto the counterrunning roller offers the additional advantagethat non-magnetic particles, for example larger lumps of toner or paperparticles that have collected in the developer mix after longeroperation are downwardly hurled into the supply chamber from the mixafter being transferred onto the counterrunning drum. The risk that suchparticles come into contact with the charge image carrier after the lastdeveloper drum and that they are entrained by the charge image carrierdue to the electrostatic forces--this potentially leading todisturbances in the printed format--is considerably reduced.

Also having a disturbance-reducing effect is that the last developerdrum having a moving direction running counter that of the charge imagecarrier is in the position to strip off such disturbing particles thatalready previously remained adhering to the surface of the charge imagecarrier and to convey them downward into the supply region together withthe developer mix. In a further advantageous embodiment of theinvention, the first and the second isorunning drum are arranged suchthat the developer mix, after passing the charge image carrier, fallsback onto the transport drum that, for example, can be fashioned as apaddle wheel and is supplied from the latter to the second isorunningdrum. The developer mix that is already somewhat depleted can thus bereturned from the first developer drum into the supply chamber andwell-blended developer mix can be transferred onto the second developerdrum.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention are shown in the drawings and shall be setforth in greater detail by way of example. Shown are:

FIG. 1 a schematic sectional view of the developer station of theinvention; and

FIG. 2 a schematic illustration of an embodiment of the developerstation comprising three developer drums.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a schematic sectional view of a developer station in anon-mechanical fast-printer means comprising the illumination means ofthe invention. A photoconductive drum 10 is arranged as charge imagecarrier in a printer means not shown in greater detail that operatesaccording to the electrophotographic principle. A charge image isapplied to this photoconductive drum in a known fashion via an exposuremeans that is controlled character-dependent and this charge image isthen inked with the assistance of the illustrated developer station. Theinking thereby ensues according to the reversal development principlewherein the regions discharged by the exposure are inked with theassistance of a developer mix 11 containing toner particles and carrierparticles. After traversing the developer station, the charge imagescomposed of colored toner particles are transferred onto paper in thestandard way.

The developer station is essentially composed of a supply chamber 12 towhich developer mix 11 is supplied via a filling aperture 13 comprisinga drum of expanded material arranged therein as metering means. Anelectromotively driven conveyor drum in the form of a paddle wheel drum14 that comprises spokelike paddles 15 for conveying the developer mix11 is situated at the floor of the supply chamber 12. The supply chamber12 is closed off from the photoconductive drum 10 by four developerdrums 16, 17, 18 and 19. These developer drums arranged along thecircumference of the photoconductive drum are situated at the tightspacing of about 1 through 2.5 mm from the surface of thephotoconductive drum and operate according to the magnetic brushprinciple. They are essentially composed of hollow cylinders constantlydriven via electromotive means, for example of aluminum having a knurledsurface and having magnet arrangements 21 arranged therein. The hollowcylinders 20 are thereby charged with a bias voltage that exhibitsapproximately the size of 20 through 50% of the charge potential at thephotoconductive drum. Given employment of a selenium photoconductivedrum having a charge potential of 400 through 1000 volts, the biasvoltage has the size of 100 through 500 volts.

Dependent on the moving direction of its hollow cylinders, the developerdrums 16, 17 and 18 are fashioned as what are referred to as isorunningdeveloper drums. Given these isorunning developer drums, the movingdirection of their hollow cylinders corresponds to the moving directionof the surface of the photoconductive drum 10 in the region of thedeveloping gap 22 formed by the hollow cylinder 20 and the surface ofthe photoconductive drum 10. The developer drum 19 is fashioned as acounterrunning developer drum wherein the hollow cylinder 20 moves in adirection opposite that of the photoconductive drum 10 in thedevelopment gap 22.

The transport of the developer mix 11 thereby ensues according to thearrows shown in FIG. 1 such that the developer mix 11 is offered to thefirst isorunning developer drum 16 from the mix sump floor of the supplychamber 12 via the paddle wheel drum 14. A metering doctor 23 therebydefines the height of the carpet of developer mix on the firstisorunning drum 16 and, thus, on the following isorunning drums 17 and18 as well. Since the developer mix has developed the charge imagecontained on the photoconductive surface three times with considerablyhigher speed than the photoconductive surface (about 1.5 times theprocess speed), namely with the assistance of the isorunning developerdrums 16, 17 and 18, the developer mix transfers from the thirdisorunning drum 18 onto the under side of the significantly slower,fourth counterrunning developer drum that is driven in the oppositedirection. A majority part of the developer mix is stripped off here bya further metering doctor 24; the remaining developer mix transported tothe surface of the photoconductive drum 10 now develops the charge imagea final time in counterrun. The spacings of the developer rollersadvantageously lie under 2.5 mm, whereby the developing gap 22 has awidth of 1 through 2.5 mm. The developer mix must be conveyed throughthis development gap 22 with optimally high density. The density of thedeveloper mix must thereby be selected such that, first, the latentcharge image is well-inked and, second, such that the surface of thecharge image carrier is not damaged as a result of excessively greatsqueezing.

In order, first, to be able to transport the developer mix with theassistance of the developer drums but, on the other hand, in order to beable to enable an agglomeration of the toner particles on the chargeimage, the surface of the developer drums, as already set forth, ischarged with a bias voltage of about 20 through 50% of the chargepotential.

An illumination means in the form of a light-emitting diode strip orlight-emitting foil 25 that is accomodated in a protective drum 26composed of a transparent, rotating plexiglass tube is situated abovethe last developer drum fashioned as a counterrunning developer drum 19.Rotation and spacing from the counterrunning developer drum 19 areadapted such that the surface of the plexiglass tube 26 is continuouslycleaned of developer mix 11 and the light exit region 27 of thelight-emitting diode ledge 25 is only slightly attenuated by adheringtoner dust. The light-emitting diode ledge thereby generates a spectrallight that approximately corresponds to the light of the charactergenerator--for, example, a LED comb.

Further, a carrier stripper drum 28 operating according to the magneticbrush principle in accord with the developer drums is situated above theillumination means, this carrier stripping drum 28 lifting the carrierparticles of the developer mix from the surface of the photoconductivedrum in collaboration with the illumination means and returning them tothe developer mix 11 via a correspondingly fashioned guide channel.

Via the light-emitting diode ledge 25, the illumination means levels thecharge image in that it illuminates the highly charged (about 400through 1000 volts) and non-toner-covered regions of the surface of thephotoconductive drum 10 and thereby discharges them down to a residualvoltage of less than 50 volts that thus corresponds to the dischargevoltage of the character generator. The adhesion of negative carrierparticles of the developer mix to the non-exposed regions of the surfaceof the photoconductive drum 10 is thus reduced, so that they can bestripped from the surface of the photoconductive drum 10 by thefollowing carrier stripper drum 28 and can be returned into thedeveloper station. Simultaneously, memory effects due to charge imageson the surface of the photoconductive drum that were not completelyquenched can be prevented by the illumination means.

The illumination means generates a largely uniform charge image on thecharge image carrier preceding the carrier stripper means and thefollowing transfer station in the printer. The charge image carriertherewith comprises a uniform residual charge voltage of about 50 volts.This "image-wise" discharging thus facilitates not only the removal ofthe carrier particles from the photoconductive drum but it also promotesthe transfer of the toner image onto the paper web in the transferstation. In order to guarantee the same penetration depth of the lightinto the surface of the charge image carrier as the light that iscontrolled in character-dependent fashion that generates the chargeimage, the light of the illumination means has approximately the samespectral structure as the light of the character generator. When forexample, a light-emitting diode comb is employed as a charactergenerator, then a similarly structured illumination means isrecommendable. Instead of a LED line, a light-emitting foil can also beemployed.

The illumination means is surrounded by a suction means that acts on theregion between the carrier stripper drum 28 and the protective drum 26via a suction channel 29 extending along the illumination means. Thissuction channel 29 is in communication with a suction blower (notillustrated here) via a suction collecting channel 30. This airextraction between the plexiglass tube (protective drum) 26 comprisingthe illumination means and the carrier stripping drum 28 generates alocal underpressure and, thus, free toner dust that is not bonded by thecharge image is collected in a container. The free toner dust can thusnot be entrained upward out of the developer station by thephotoconductive drum 10. Carrier particles stripped by the carrierstripping drum 28 and developer mix stripped by the metering doctor 24of the counterrunning developer drum are returned into the supplychamber 12 via baffle plates 31.

An emptying aperture 32 via which used developer mix is suctioned offafter a defined operating time is situated at the floor of the supplychamber 12.

Three developer drums 33, 34 and 35 are arranged in the embodiment ofthe developer station shown in FIG. 2. The developer drums 33 and 34 area matter of isorunning developer drums; the developer drum 35 is amatter of a counterrunning developer drum. In this exemplary embodiment,the developer mix is first offered to the photoconductive drum 10 viathe isorunning developer drum 33. The developer mix is then returned tothe paddle wheel drum 14. The developer mix that is already somewhatdepleted can thereby be enriched again with new developer mix and thisnewly enriched developer mix is then offered to the surface of thephotoconductive drum again via the developer drum 34. It is therebynecessary to also allocate a metering doctor 36 to the developer drum34. The counterrunning developer drum 35 arranged following thereuponcorresponds in function to the counterrunning developer drum 19.Although other modifications and changes may be suggested by thoseskilled in the art, it is the intention of the inventors to embodywithin the patent warranted hereon all changes and modifications asreasonably and properly come within the scope of their contribution tothe art.

    ______________________________________                                        List of Reference Characters                                                  ______________________________________                                        10           photoconductive drum                                             11           developer mix                                                    12           supply chamber                                                   13           filling aperture                                                 14           paddle wheel drum                                                15           paddle                                                           16           isorunning developer drum                                        17           isorunning developer drum                                        18           isorunning developer drum                                        19           counterrunning developer drum                                    20           hollow cylinder                                                  21           permanent magnet                                                 22           development gap                                                  23           metering doctor                                                  24           metering doctor                                                  25           LED ledge                                                        26           protective drum                                                  27           light exit region                                                28           carrier stripper drum                                            29           suction channel                                                  30           suction collecting channel                                       31           baffle plate                                                     32           emptying aperture                                                33           isorunning developer drum                                        34           isorunning developer drum                                        35           counterrunning developer drum                                    36           metering doctor                                                  ______________________________________                                    

We claim:
 1. A developer station in an electrophotographic means for thedevelopment of charge images applied to charge image carriers (10) withthe assistance of a developer mix (11) is taken from a supply chamber(12) in a developer station by a transport drum (14) and is transportedto developer drums (16 through 19) that rotate in opposite directionsrelative to one another and ink the charge image carrier (10),characterized in that the developer mix (11) is first transported to afirst isorunning developer drum (16) moving in the same direction as thecharge image carrier at a first development gap defined between thefirst developer drum and the charge image carrier (22); in that furtherdeveloper drums (17, 18) moving in the same direction as the chargeimage carrier at further development gaps are provided; and in that thedeveloper mix (11) is then forwarded to a counterrunning developer drum(19) following said further developer drums in a moving direction of thecharge image carrier (10) and moving in a direction opposite that of thecharge image carrier (10) at a final development gap (22) and means forconducting said developer mix from the counterruning developer drum backinto the supply chamber (12), the isorunning developer drums (16, 17,18) have a circumferential speed that is noticeably higher than thecircumferential speed of the charge image carrier (10).
 2. A developerstation according to claim 1, characterized in that the first isorunningdeveloper drum (16) and a second of the further isorunning developerdrums (17) arranged in the moving direction of the charge image carrierare arranged such that the developer mix (11) is again supplied to thetransport drum (14) after passing the first development gap (22) and istransported from the transport drum to the second isorunning drum (17).3. A developer station according to claim 1, characterized in that thetransport drum (14) is fashioned as a paddle wheel drum having spokescomprising individual paddles (15).
 4. A developer station according toclaim 1, characterized in that an illumination means (25, 26) forilluminating the charge image carrier (10) is provided following thefurther developer drums in the moving direction of the charge imagecarrier (10).
 5. A developer station according to claim 4, characterizedin that the illumination means is provided.
 6. A developer stationaccording to claim 5, characterized in that the protective drum (26) isarranged at a tight spacing relative to the counterrunning developerdrum (19) so that the counterrunning developer drum (19) cleans theprotective drum (26) of adhering developer mix (11).
 7. A developerstation according to claim 1, characterized in that a carrier stripperdrum means (28) for picking up carrier parts of the developer mix isarranged in the moving direction of the charge image carrier (10)following the further developer drums (16 through 19).
 8. A developerstation according to claim 1, characterized in that a suction means (29,30) that extracts parts of the developer mix that are not bonded to thecharge image carrier is provided following the further developer drumsin the moving direction of the charge carrier.
 9. A developer stationaccording to claim 8, characterized in that an illumination means forilluminating the charge image carrier is provided following the furtherdevelopment drums in the moving direction of the charge image carrier;acarrier stripper drum means for picking up carrier parts of thedeveloper mix is arranged following the further development in themoving direction of the charge image carrier; and the suction means isarranged between the illumination means and the carrier stripper drummeans (28).